Currently few processes will allow the movement of the boundary formed between the two fluids to be monitored continuously.
The processes in existence are basically nuclear, electrical and seismic.
It is possible to sink inspection wells between the injection well and the production well and from these produce a series of diagraphs of the resistivity or nucleation. These show the variation in saturation of the fluids being monitored, usually salt water around the inspection well, normally within a range of about a meter. The extent of investigation is limited and it is necessary to generate a large number of inspection wells in order to track the movement of the boundary, the injection well being located at several hundreds of meters from the production well. The multiplication of the quantity of inspection wells is very expensive.
It is desirable that the two fluids have sufficiently different resistivities, which is the case for salt water and hydrocarbons.
This difference is thus approximately 1 to 100, if not the measurements are insignificant and their interpretation becomes very difficult.
A process of seismic inspection in four dimensions includes the use of repeated measurements of the time taken by sound waves to cross a predetermined thickness of the geological formations located between a sound wave generator source at the surface or below it and microphones installed at various depths. The velocity of propagation of the sound waves in the rocks is a function of several factors notably their porosity and the type of fluids contained within them and the pressure. The installation of the equipment needed to perform the measurements is very complicated especially if the oil bearing strata concerned are beneath the sea-bed.
It is desirable that the two fluids have sufficiently different densities so that the measurements have a high sensitivity thus making the interpretation simple. Such is the case if one of the fluids is a gas and the other a liquid.